1. Field of the Invention
The present invention relates to a wafer conveyor used in a semiconductor manufacturing apparatus in which dust scattering by conveyance has to be avoided, a vacuum conveyor in which the scattering of lubricating oil has to be avoided, a conveyor in a high temperature atmosphere and other conveyors used in a field required to convey an object without contact. Particularly, the present invention relates to an AC magnetic floating conveyor and its operation method for conveying a conductive floating body of nonmagnetic metallic material which is floated above a series of AC electromagnets and on which an object to be conveyed is placed.
2. Description of the Related Art
As a bearing used in a mechanism for conveying or moving an object, for example, such as a semiconductor wafer in a semiconductor manufacturing process line in which dust and oil scattering needs to be avoided, a so-called magnetic bearing using permanent magnets or electromagnets is employed. This employs the magnetic attraction or repulsion acting between magnets or between a magnet and magnetic material to float an object to be conveyed or a support object through space without contacting.
A number of patent applications have been filed for various inventions related to the magnetic conveyor and many of the inventions have been put into practice.
Of these inventions, magnetic bearing using an electromagnet has a merit that electric power necessary for operation of the magnetic bearing is small. However, since it is necessary to stabilize the inherently unstable attraction of the electromagnet by electric control, the magnetic bearing has drawbacks that a high-performance control circuit apparatus is required and many secondary devices such as a gap sensor are required. Specifically, when a conveyance distance is long, it is necessary to provide these control apparatuses over the whole conveyance line and accordingly there are many structural difficulties and economic problems.
As a countermeasure for avoiding such difficulties, a method is adopted in which a necessary control circuit and sensor are mounted on an object to be conveyed. In this case, however, it is necessary to provide a power supply unit such as a battery in the object and accordingly there occurs such a problem as that a charging system or a power supply system is further required.
On the other hand, if magnetic repulsion is utilized, the control apparatus as described above becomes in principle unnecessary and such an apparatus utilizing magnetic repulsion has a large advantage as a conveying mechanism for a long distance
However, it is considered that a repulsive type floating mechanism made up only of permanent magnets cannot be realized in practice and the degree of freedom of at least one or more axes needs to be stabilized by the control circuit described above.
On the contrary, it is confirmed in principle that an induction repulsive type magnetic floating mechanism using an AC electromagnet can float a floating body and support it stably by devising a suitable shape of the floating body made of inductive material.
FIGS. 18 to 21 show an example of the induction repulsive type magnetic floating mechanism. In the figures, numeral 01 denotes an AC electromagnet and numeral 02 denotes a floating body to be conveyed. Light and high-conductive material such as aluminum is suitable for the floating body 02. An object to be conveyed is usually placed on the floating body 02. In the figures, when a single-phase AC current indicated by 04 of FIG. 20 is supplied to the AC electromagnets 01, an alternating magnetic field is generated above the electromagnets. Since the floating body is present in the magnetic field, an alternating current called an eddy current is generated in the aluminum material forming the floating body.
A magnetic field generated by the eddy current repels the magnetic field generated by the electromagnets. Accordingly, a floating force shown by F1 of FIGS. 18 and 19 acts on the floating body by this repulsion. When three phase alternating currents indicated by 05, 06 and 07 of FIG. 21 are supplied to three-phase electromagnets 03 of FIGS. 18 and 19, a moving force F2 of FIGS. 18 and 19 acts on the floating body 02 and the floating body is conveyed.
The foregoing is an example of the conventional AC magnetic floating conveyor of the induction repulsive type.
The above induction repulsive type magnetic floating conveyor requires the electromagnets for floatation and the electromagnets for movement separately and further requires power supplies separately. Accordingly, the whole mechanism is large and extremely expensive.
Further, in the AC magnetic floating conveyor, when the floating body is moved from a position to another position on a conveyance line, all the coils of the electromagnets disposed along the conveyance line are supplied with an AC current to be excited. In this case, because of extremely large inductance of the coils, there arises a problem that a power supply with large capacity is required in order to flow a predetermined current against this extremely large inductance. Furthermore, since electromagnets which do not contribute actually to float the floating body are also excited, there is a problem that power efficiency is very low and power is consumed very uselessly.
In a semiconductor process line, since it is necessary to transfer wafers between a conveyance path and a process apparatus, the accuracy of the position where the floating body is stopped is important for the transport of semiconductor wafers by the AC magnetic floating conveyor.
However, it is difficult to control a fine stop position of the floating by the known conventional method of gradually reducing the strength of the magnetic field component for movement of the floating body to simply zero because of an influence from the inertial force of the movement of the floating body. Accordingly, inaccuracy of stopping positions has been a problem.